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You’re not understanding the gain correctly....
The full well is always the same, 20000 or whatever it is. When we expose, the number of photons each photosite receives doesn’t change (well, it does, but that’s a different matter altogether!). What happens is that the amplifier scales the output to fit between 0 and 4095 (note that this is actually 4096 values, which equals 2 to the 12th power) by different amounts, depending on what gain setting we choose.
It doesn’t matter whether you operate in 12-bit or 16-bit, so long as you’re comfortable with the calculation.
You could expose for two hours, but it would likely fill up the wells and full wells are full wells regardless of the gain. A full well would be represented by 4095, multiplied by 16 for writing to FITS.
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